1. Field of the Invention
The present invention relates to gas-discharge lamp controllers, and more particularly to gas-discharge lamp controllers capable of providing preheating time setting for gas-discharge lamps.
2. Description of the Related Art
In supplying power to gas-discharge lamps, electronic ballasts are widely adopted to keep the lamp current stable.
To increase the lifetime of gas-discharge lamps, the electronic ballasts should start with a preheating phase to pre heat the lamps, enter an ignition phase after the preheating phase to ignite the lamps, and then settle to a steady phase. Of the three phases, the preheating phase is required to have a precise time duration to facilitate the ignition of the lamps and thereby prolong the lifetime of the lamps.
A prior art solution for controlling the time duration of the preheating phase is utilizing a current source inside a gas-discharge lamp controller to charge an external capacitor, and as the voltage on the external capacitor, increasing from a low voltage, reaches a reference voltage which is provided in the gas-discharge lamp controller and independent of the supply voltage of the gas-discharge lamp controller, the preheating phase is ended. Please refer to FIG. 1, which shows a block diagram of part of a ballast circuit, including a prior art gas-discharge lamp controller and an external capacitor. As can be seen in FIG. 1, the gas-discharge lamp controller 100, coupled with a capacitor 110, including a current source 101 and a comparator 102.
The current source 101, coupled to a supply voltage VCC, is of small current and used to charge the capacitor 110 to generate a slowly increasing voltage VC. The comparator 102 is used to compare the slowly increasing voltage VC with a reference voltage VREF, the reference voltage VREF being independent of the supply voltage VCC. As the slowly increasing voltage VC reaches the reference voltage VREF, an output signal SPHE of the comparator 102 will change state from low to high to indicate the end of the preheating phase.
As a typical example, the time duration of the preheating phase is around 1 second. To minimize the production cost, the capacitance of the external capacitor 110 is required to be as small as possible, as such, the current source 101 has to be rated at a small current. However, the variance of this small current is tending to be large due to two causes—device variations and the supply voltage VCC variations. When it comes to a small current, the widths of the related MOSFETs have to be narrow, so the small current is very sensitive to device variations; and when the supply voltage VCC becomes higher/lower, the current source 101 is inclined to follow, which will make the time duration of the preheating phase shorter/longer. As such, this kind of design can not provide a fixed, precise preheating time for the gas-discharge lamps.
In view of the cons of the prior art design, the present invention proposes a novel topology of a gas-discharge lamp controller capable of providing a precise preheating time without adding any extra pin.